Calgary police investigating the disappearance of a five-year-old boy and his grandparents say there was a violent incident in the home where the three were last seen five days ago.

Staff-Sgt. Doug Andrus says police found forensic evidence at the home, but would only disclose they believe it was from someone in medical distress.

The evidence has been sent to a police lab for analysis.

The investigation has also turned to locating the driver of a vehicle who may have more information on the whereabouts of five-year-old Nathan O'Brien and his grandparents, Alvin and Kathy Liknes.

Police say a truck was seen in the Parkhill area the night of the disappearance, and Andrus said the vehicle was seen travelling the street several times.

Police describe the truck as a green Ford F150 from the late '80s or early '90s, kept in very good condition.

Police aren't calling the driver of the truck a suspect but believe he may have more information about the three missing people.

The trio were reported missing Monday and police believe their disappearance is suspicious.

The boy had been at his grandparents' on a sleepover after the couple held an estate sale at their home on the weekend. Police have said there was evidence the three may have been forced from the home.

Andrus said Thursday investigators still don't have any suspects they can name, but confirmed that police has spoken to approximately 75 people who had attended the estate sale.

Police had asked anyone who attended the sale and bought something to bring a photo of the item to a community centre Thursday and speak with investigators. Six tables manned by police officers with notepads were set up inside for interviews.

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Nathan O'Brien, Grandparents Missing

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Mourners have placed flowers, notes and teddy bears at an impromptu memorial for Nathan O'Brien and his grandparents, Alvin and Kathryn Liknes, at the Liknes home in Calgary, Alta., Tuesday, July 15, 2014.

Flowers and teddy bears are shown at an impromptu memorial for Nathan O'Brien and his grandparents, Alvin and Kathryn Liknes, at the Liknes home in Calgary, Alta., Tuesday, July 15, 2014. A man who has been under investigation in the disappearance of a Calgary boy and his grandparents has been formally charged with their murders.

Bobbi-Jo Zimmer, from High River, Alta., and her children, left to right, Tayvon, two, Braylon, seven, and Dawson, nine, place flowers at an impromptu memorial for Nathan O'Brien and his grandparents, Alvin and Kathryn Liknes, at the Liknes home in Calgary, Alta., Tuesday, July 15, 2014.

Mourners have placed flowers, notes and teddy bears at an impromptu memorial for Nathan O'Brien and his grandparents, Alvin and Kathryn Liknes, at the Liknes home in Calgary, Alta., Tuesday, July 15, 2014.

Kelley MacAuley observes a memorial after placing flowers at an impromptu memorial for Nathan O'Brien and his grandparents, Alvin and Kathryn Liknes, at the Liknes home in Calgary, Alta., Tuesday, July 15, 2014.

Jennifer Kohut, centre, embraces her sons Axton, left, three, and Cadon, five, after placing flowers and toys at an impromptu memorial for Nathan O'Brien and his grandparents, Alvin and Kathryn Liknes, at the Liknes home in Calgary, Alta., Tuesday, July 15, 2014.

Douglas Garland is escorted into a Calgary police station in connection with the disappearance of Nathan O'Brien and his grandparents in Calgary, Alta., Monday, July 14, 2014.

Douglas Garland is escorted into a Calgary police station in connection with the disappearance of Nathan O'Brien and his grandparents in Calgary, Alta., Monday, July 14, 2014.

Douglas Garland is escorted into a Calgary police station in connection with the disappearance of Nathan O'Brien and his grandparents in Calgary, Alta., Monday, July 14, 2014.

Douglas Garland is escorted into a Calgary police station in connection with the disappearance of Nathan O'Brien and his grandparents in Calgary, Alta., Monday, July 14, 2014.

Calgary Police Chief Rick Hanson says first and second degree murder charges are being filed in connection with the disappearance of Nathan O'Brien and his grandparents at a news conference in Calgary, Alta., Monday, July 14, 2014.

Calgary Police Chief Rick Hanson says first and second degree murder charges are being filed in connection with the disappearance of Nathan O'Brien and his grandparents at a news conference in Calgary, Alta., Monday, July 14, 2014.

Calgary Police Chief Rick Hanson says first and second degree murder charges are being filed in connection with the disappearance of Nathan O'Brien and his grandparents at a news conference in Calgary, Alta., Monday, July 14, 2014.

People comfort each other at a candlelight vigil for missing Calgarians Nathan O'Brien, five, and his grandparents Alvin and Kathryn Liknes in Calgary, Alta., Thursday, July 10, 2014.

People comfort each other at a candlelight vigil for missing Calgarians Nathan O'Brien, five, and his grandparents Alvin and Kathryn Liknes in Calgary, Alta., Thursday, July 10, 2014.

A photo collage of the Liknes and O'Brien family is displayed as neighbors, friends and relatives hold a candlelight vigil for missing Calgarians Nathan O'Brien, five, and his grandparents Alvin and Kathryn Liknes in Calgary, Alta., Thursday, July 10, 2014.

Rod O'Brien, centre, tears up as he looks at a photo collage of the Liknes and O'Brien family during a candlelight vigil for missing Calgarians Nathan O'Brien, five, and his grandparents Alvin and Kathryn Liknes in Calgary, Alta., Thursday, July 10, 2014.

A photo collage of the Liknes and O'Brien family is displayed as neighbors, friends and relatives hold a candlelight vigil for missing Calgarians Nathan O'Brien, five, and his grandparents Alvin and Kathryn Liknes in Calgary, Alta., Thursday, July 10, 2014.

Rod O'Brien, centre, father of Nathan O'Brien, attends with neighbors, friends and relatives, a candlelight vigil for missing Calgarians Nathan O'Brien, five, and his grandparents Alvin and Kathryn Liknes in Calgary, Alta., Thursday, July 10, 2014.

Police investigators comb a hay field north of Airdrie, Alta., Monday, July 7, 2014, looking for clues to the disappearance of three people.

Police investigators use an inflatable raft in a slough north of Airdrie, Alta., Monday, July 7, 2014, looking for clues to the disappearance of three people.

Police investigators prepare to search a hay field north of Airdrie, Alta., Monday, July 7, 2014, looking for clues to the disappearance of three people.

Police investigators prepare to search a hay field north of Airdrie, Alta., Monday, July 7, 2014, looking for clues to the disappearance of three people.

Police investigators prepare to search a hay field north of Airdrie, Alta., Monday, July 7, 2014, looking for clues to the disappearance of three people.

Police investigators prepare to search a hay field north of Airdrie, Alta., Monday, July 7, 2014, looking for clues to the disappearance of three people.

Police investigators at a rural residence north of Airdrie, Alta., Saturday, July 5, 2014, looking for clues to the disappearance of three people.

Police investigators comb a hay field north of Airdrie, Alta., Saturday, July 5, 2014, looking for clues to the disappearance of three people.

Police investigators comb a hay field north of Airdrie, Alta., Saturday, July 5, 2014, looking for clues to the disappearance of three people.

Police investigators comb a hay field north of Airdrie, Alta., Saturday, July 5, 2014, looking for clues to the disappearance of three people.

Police investigators comb a hay field north of Airdrie, Alta., Saturday, July 5, 2014, looking for clues to the disappearance of three people.

Police investigators comb a hay field north of Airdrie, Alta., Saturday, July 5, 2014, looking for clues to the disappearance of three people.

Police investigators at a rural residence north of Airdrie, Alta., Saturday, July 5, 2014, looking for clues to the disappearance of three people.

Police investigators at a rural residence north of Airdrie, Alta., Saturday, July 5, 2014, looking for clues to the disappearance of three people.

Rod O'Brien wipes his tears while holding a butterfly made by his son as he speaks to the media about his son Nathan O'Brien and his grandparents Alvin and Kathryn Liknes during a press conference in Calgary on Wednesday, July 2, 2014.

Rod and Jennifer O'Brien show their emotions as they speak to the media about their son Nathan O'Brien and his grandparents Alvin and Kathryn Liknes during a press conference in Calgary on Wednesday, July 2, 2014.

Rod and Jennifer O'Brien left, look on as Jeff Liknes, second from right, speaks to the media about his nephew Nathan O'Brien and his parents Alvin and Kathryn Liknes during a press conference in Calgary on Wednesday, July 2, 2014.

Jennifer O'Brien holds a piece of her son's clothing while speaking to the media about her son Nathan O'Brien and his grandparents Alvin and Kathryn Liknes during a press conference in Calgary on Wednesday, July 2, 2014.

Jennifer O'Brien holds a piece of her son's clothing while speaking to the media about her son Nathan O'Brien and his grandparents Alvin and Kathryn Liknes during a press conference in Calgary on Wednesday, July 2, 2014.

Rod O'Brien wipes his tears while holding a butterfly made by his son as he speaks to the media about his son Nathan O'Brien and his grandparents Alvin and Kathryn Liknes during a press conference in Calgary on Wednesday, July 2, 2014.

Rod and Jennifer O'Brien show their emotions as they speak to the media about their son Nathan O'Brien and his grandparents Alvin and Kathryn Liknes during a press conference in Calgary on Wednesday, July 2, 2014.

Nathan O'Brien is described as three feet tall, curly blonde hair, brown eyes, and was last seen wearing peach-coloured shorts and a blue-striped hoodie.

Alvin Liknes is six feet tall, 200 pounds with grey and blond hair and was wearing black shorts. Alvin has a distinctive walk.

Calgary police investigators check out the home where five-year-old Nathan O'Brien and his grandparents Alvin and Kathryn Liknes disappeared, in Calgary, Alta., Wednesday, July 2, 2014.

Nathan O'Brien and his grandmother Kathy Liknes are shown in a Calgary Police Service handout photo. Calgary's police chief questions whether a reward would help the investigation into the disappearance of a five-year-old boy and his grandparents.

Police tape cordons off the home where five-year-old Nathan O'Brien and his grandparents Alvin and Kathryn Liknes disappeared, in Calgary, Alta., Wednesday, July 2, 2014.

Calgary police investigators check out the home where five-year-old Nathan O'Brien and his grandparents Alvin and Kathryn Liknes disappeared, in Calgary, Alta., Wednesday, July 2, 2014.

Calgary police investigators check out the home where five-year-old Nathan O'Brien and his grandparents Alvin and Kathryn Liknes disappeared, in Calgary, Alta., Wednesday, July 2, 2014.

Police have said there are no custody issues with the family — the boy's parents are married and he is their biological son. Police have also said officers have no history with the family. All of the family's vehicles have been accounted for.